Magnesium base alloys



2,788,272 MAGNESIUM BASE ALLOYS Derek J. Whitehead and James H. T. Petch, Clifton Junction, Manchester, England, assignors to Magnesium Elektron Limited, Manchester, England, a British com- P y N Drawing. Application April 7, 1955, Serial No. 500,016 Claims priority, application Great Britain April 26, 1954 4 Claims. (Cl. 75168) This invention relates to magnesium base alloys. The invention is more particularly concerned with magnesium base alloys for use in the cast form. The alloys may, however, be advantageously used in the wrought form.

In the specification of British patent application No. 13,845 of 1950, now Patent No. 733,221, granted Oct. 26, 1955, magnesium base alloys (known as ZTl alloys) are described having exceedingly high resistance to creep at elevated temperatures. This British patent is commonly assigned to Magnesium Elektron Limited, along with the present case. These alloys contain zirconium between 0.1 and 0.9%, thorium between 1 and 6%, and zinc between 0.5 and the preferred proportion of thorium being between nine-tenths and three times the percentage of zinc, but not exceeding nine-tenths of the zinc content by more than two and three quarters percent, the thorium and zinc together not exceeding ten percent. In our continued search for alloys having a high resistance to creep at elevated temperatures we have experimented with additions of manganese to alloys containing thorium but have found that this tends to produce somewhat coarse grained alloys with correspondingly low mechanical properties at room temperature. With a view to overcoming these disadvantages we therefore experimented with various grain refining treatments but without success. It was known that additions of zirconium would efiect grain refinement of magnesium base alloys in the absence of elements such as aluminium, manganese, silicon, which combine therewith to form insoluble particles. This principle was enunciated in the specification of British Patent No. 511,137 and has been subsequently confirmed by various investigators. The solubility of all these elements in magnesium containing zirconium has hitherto been regarded as negligible and in the case of aluminium, silicon and iron does not exceed about 0.01%.

We have, however, now found that manganese differs from the other interfering elements in that although additions of zirconium to alloys containing manganese do usually result in precipitation of insoluble particles containing manganese and zirconium it is possible to retain a small but useful proportion of each in the alloy and when this is applied to magnesium alloys containing thorium and zinc an unusually high creep resistance at elevated temperatures can be achieved. Both the manganese and zirconium contribute to the enhanced creep resistance, and the presence of the manganese results in alloys of appreciably better corrosion resistance than would have otherwise been obtained without it. It appears that for a given percentage of zirconium in solution there is a certain maximum quantity of manganese which can be tolerated. Thus, for example, if the alloy contains only 0.4 percent zirconium it is possible to have up to about 0.25 percent manganese also present in solution or if the alloy contains only 0.1 percent zirconium it is then possible for the alloy to contain up to 0.5 percent manganese in solution. This appears to be true whether zinc is present or not.

2,788,272 Patented Apr. 9, 1957 According to the present invention we provide a magnesium base alloy consisting apart from impurities of the following alloy constituents:

Percent Zirconium ..from 0.2 to 0.7 Manganese from 0.10 to 0.3 Zinc from 0.5 to 5.0 Thorium from 1.0 to 6.0

Preferably the quantity of zirconium is between 0.3 and 0.6 percent and the manganese at least 0.15 percent. It will be appreciated that the proportions of zirconium and manganese are approximately in inverse relation to each other. We have, however, found that the proportion of thorium should be preferably from 0.6 to three times the percentage of zinc and the combined percentages of thorium and zinc should preferably not exceed 10%. We find that particularly good results are obtained when the content of thorium is from 2.6 to 6.0 percent, zinc 1.0 to 5.0 percent and the thorium is from 2+six-tenths of the zinc content to 2.5 times the zinc content. The thorium and zinc together do not exceed 10%. The thorium content can usefully be greater than 1.5 plus nine-tenths of the zinc content.

The following approximate composition (known as ZTM alloy) has been found to be particularly suitable: Alloy ZT M composition Percent Thorium 3.5 Zinc 2.0 Zirconium 0.5 Manganese 0.2

Analysis Percent Creep Strain at S tress tons per Sq. inch Alloy Th Zn Zr 500 hrs.

Temperature of test, 350 C.

These tests were made on sand cast test bars made according to British Standard Specification L101 and heat treated at 350 C. for 16 hours and subjected to test at 350 C. It can be seen from the first three test results tabulated above that under these conditions of test, ZTM alloy shows considerably enhanced creep resistance for periods of exposure in excess of 100 hours when compared with ZTl alloy of substantially equal thorium and zinc contents. In addition, the last two test results show that ZTM alloy with a slightly higher thorium content aud lower zinc content possesses superior long term creep resistance at a stress of 1.25 tons per square inch to ZTl alloy at a stress of only 0.75 tons per square inch.

The improved corrosion resistance of ZTM alloy compared with ZTl alloy is shown by the following tests which were carried out by total immersion of the samples for 28 days in 3% salt solution saturated with magnesium hydroxide.

Thealloys-of the present invention may contain small quantities of other metals for example:

Rare earth metals up to 4% (more particularly 0.1 to 0.7 per- Beryllium up to 0.1%.

Calcium up to 0.2%.

Mercury up to 3%.

Lead, up to 1%.

Thallium up to'1%.

Lithium up to 1%.

Indium up to 1%.

Tellurium up to 0.1%.

Iflead is included the proportion may be from 0.01 to 1.0 percent preferably 0.025 to 0.5 e. g. 0.25%.

The beneficial effect of lead on the creep resistance of ZTM alloy at 350 C. is shown by the following test results. Corrosion tests included in the same table indicate that lead has a negligible efiect on the corrosion resistance of ZTM when added in amounts of up to 0.15%.

Corrosion rate mgmJsq cm. day (28 days total immersion Percent Creep strain at 1.25 t. s. i. at 350 C. at- Th Zn Zr Mn Pb in 3% NaCl 100 500 1,000 solution satuhrs. hrs. hrs. rated with S. 2 2. 0 0. 41 0 l7 0. 2O 0. 48 0. S2 1. 1 3. 1 2. 0 0. 40 0 18 0. 025 0. 17 0. 45 0. 74 0. 7 3.0 2.0 0.41 0 18 0.055 0,8 5. 0 2.0 0. 40 0 18 0. 096 0. 0.25 0.8 3.0 2.0 0. 39 0 17 0. 154 0.10 0. 22 0. 3S 1. 2

. 4 We claim: 1.. A magnesium base alloy consisting apart from impurities of the following alloy constituents:

Zirconium from 0.2 to 0.7%. Manganese from 0.10 to 0.3%. Zinc from 0.5 to 5.0%. Thorium from 1.0 to 6.0%. Magnesium balance.

2. An alloy as claimed in claim 1, wherein the quantity of zirconium is between 0.3 and 0.6 percent and the manganese is at least 0.15 percent, thorium 2.6 to 6.0 percent, zinc is 1.0 to 5.0 percent, the thorium percentage is from two plus six-tenths of the zinc percentage to 2.5 times the zinc percentage, and the thorium and zinc together do not exceed ten percent.

3. An alloy as claimed in claim 1 having a lead content between 0.01 percent and 1.0 percent.

4. An alloy as claimed in claim 1 having a lead content between 0.025 and 0.5 percent.

References Cited in the file of this patent UNITED STATES PATENTS 1688110-. Aug. 24,1951 Ies'sup' July' 22, 1952 

1. A MAGNESIUM BASE ALLOY CONSISTING APART FROM IMPURITIES OF THE FOLLOWING ALLOY CONSTITUENTS: 